Scale outbreaks found in blueberry fields
Late last week some observant scouts brought in samples of scale-infested blueberry shoots to our MSU Extension meeting in West Olive, Michigan. On visiting some fields yesterday, it is clear that scale populations are very high in some hot-spots in the Holland and West Olive regions of the state, with some of last year’s growth having high scale abundance. Growers and scouts should be on the lookout for scale throughout the state when they are walking their fields, to check and see whether they have similar infestations.
The scale we are finding are either dark black or tan colored (see photo) and are mostly on last years’ (green) shoots. On shoots growing diagonally or horizontally, they tend to be on the lower side of the shoot in the shade, making them a little harder to see. Some infestations have only a few scales per shoot, where others are well-covered with scale. One tell-tale sign we observed yesterday was a shiny film of honeydew that the scales emit (similar to aphids) and these drop onto the leaves beneath the scales. This can help you find scale infestations in a bush, but this honeydew will be washed off by rain and may take a few days to build up again. Scale colonies were found on one-year-old shoots in all parts of the bushes we sampled, but they were variable across the field. This pest cannot be seen without walking through fields and looking closely on the bushes.
This scale species found this season is thought to be Lecanium scale (samples have been submitted to MSU Diagnostic Services for confirmation), a soft scale pest of many deciduous plants. This is a different species from the Putnam scale that can get onto fruit at harvest-time. Scales have an outer covering that they make, and the small soft scales live underneath this protection. Scale feeding creates honeydew that can act as a substrate for sooty mold, and their removal of sap by feeding can also weaken shoots, killing them if populations are high enough. Vigorous and healthy blueberry bushes can tolerate some scale infestation, but if high populations of Lecanium scale are found, control programs should be considered.
Last year was a bumper year for Lecanium scale in many parts of Michigan, and so it looks as if the 2009 crawlers settled down and have survived the winter in some blueberry fields. The Lecanium scale has one generation per year, and is currently growing through molts from the overwintered scale (small black one in the photo), to a larger tan scale, and will then molt again before becoming a mature female. This stage will lay eggs under the scale, and the eggs then hatch and the crawlers disperse from the protective covering to find new places to feed on the bush.
Natural enemies usually regulate scale populations and prevent outbreaks of these pests, but growers with high populations this season should consider chemical control options. It is too late now for oil applications to suffocate the scale that can be used in the early spring dormant timing. There is one insecticide with high activity on Lecanium scale, a Valent product called Esteem. This insecticide is registered for use in blueberry at 5 oz/acre for Lecanium scale control, and the details are described in a supplemental label (available at http://www.cdms.net). Esteem is an insect growth regulator that acts by disrupting the scale’s normal molting. Application of Esteem should be planned soon, to disrupt their development before they reach the mature adult stage. Esteem is also active on eggs of fruitworms, so there may be an opportunity to combine scale and early fruitworm control. Esteem’s effects may take some time to see, but with good coverage and timely application, it should prevent the scales reaching the stage where crawlers will be produced.
Most other insecticides that are active on scale are used at the crawler stage, later in the season, because the scale’s waxy covering protects the scale inside. Crawlers are much more vulnerable to insecticides than the mature scales, and we will be monitoring the infested fields to identify when we see crawler emergence. This can be done in your own fields by placing some double-sided sticky tape near to scales on infested shoots, and checking with a hand lens until you see tiny dots (the young crawlers) on it.